1. Field of the Invention
The present invention generally relates to methods and systems for well intervention operations in subsea wells, and more particularly to a system and method for deploying a subsea well intervention system from a surface vessel to a subsea tree.
2. Background
A subsea well intervention system includes a subsea intervention package connected to the subsea tree and a spoolable compliant guide (SCG) that will link the vessel to the subsea intervention package and allow conveyance of the intervention tools into the wellbore. The subsea intervention package, including the emergency disconnection package, the lubricator and the upper and lower intervention packages, is usually deployed from the vessel to the tree using guide wires. Some sections of the SCG are made buoyant by construction in order to provide a shape for allowing structural compliance when exposed to the environmental effects of the waves and current. Therefore, it requires a dead weight to be lowered to the seabed in a usually congested area, for its bottom connector extremity to be lowered through the water column. It also requires additional installation aids (e.g., remotely operated vehicle (ROV), Active Heave Compensation, winch, buoyancy, guide wires, etc.) and critical path time to align the bottom connector with the lubricator, and make-up and test the subsea connection.
U.S. Pat. No. 4,730,677 describes a method and system for maintaining and servicing subsea well. The system utilizes a flexible riser to link the surface vessel to the X-mas tree. The lower intervention package only is connected on surface to the flexible riser. However, the means of deployment is via the flexible riser powered reel, and the weight of the intervention package applied to the flexible riser is controlled via buoyancy modules attached to the lower intervention package. For deep water, the structure of the flexible riser will require several layers of armor wires and therefore the weight of the flexible riser will increase rapidly with the water depth. The application of this deployment method may therefore be limited in terms of water depth, for example, as:
The buoyancy required along the flexible riser will increase significantly with the weight of the flexible.
The tension capacity of a typical powered reel is limited to approximately 25 Te.
The flexible riser has a limited crushing resistance on the reel when submitted to high tension loads.